The key to the achievement is having the DPS, or at least one or two strong AOE capable DPS, remain on the stair for the entire first phase of the encounter. The undead that enter the chamber have low HP and must simply be tagged to prevent them from getting down to the floor. The other issue is the larger elite undead that spawn occasionally. Only a small number will spawn during the entire event, and the best defense is to let your tank know that they've appeared so they can move up the stair case to grab them. This is easily accomplished by a Warrior or Druid tank with a charge ability, and the elites are not very strong overall. So long as at least one dedicated DPS continues to tag all the incoming undead and the tank becomes aware of the presence of the larger elites, the achievement will be yours.